An electrophotographic printer forms an electrostatic latent image on a photosensitive drum by light according to an image signal, develops the latent image by selectively attracting toner thereto, and then transfers the developed image onto a paper to obtain a print image. As a light source for forming the electrostatic latent image, a laser and a light-emitting diode array are widely used. In particular, since a light source constituted by the light-emitting diode array does not need a long optical path unlike the laser-type light source, it is suitable for small-sized printers and large-sized printing. Demand has been mounting recently on light-emitting diode arrays of higher precision and emission, as printing has been becoming faster with higher image quality, and as printers have been becoming smaller.
Widely known is a light-emitting diode having a pair of electrodes on an upper surface (light-extracting side) and a lower surface of a semiconductor substrate with a light-emitting portion. When voltage is applied between the upper-surface electrode and the lower-surface electrode of a light-emitting diode having such a structure, electric current flows in a direction perpendicular to the semiconductor substrate, so that the light-emitting portion emits light due to the recombination of pairs of electrons and holes therein. Therefore, the largest light output is obtained in part of the light-emitting portion directly under the upper-surface electrode. However, because of reflection and absorption by the upper-surface electrode, light generated in that part cannot be efficiently taken out. Hence, conventional light-emitting diodes disadvantageously fail to provide enough light output particularly as light sources of high-resolution printers of 600 dpi, 1200 dpi, etc. needing small light-extracting portions.
JP 2000-323750 A discloses a light-emitting diode array comprising light-emitting diodes each having a light-emitting portion sandwiched by an anode and a cathode on both sides, each light-emitting diode having a current diffusion layer formed directly under a light-emitting surface of the light-emitting portion, and a current-blocking layer formed directly under the anode, wherein electric current from the anode is guided by the current-blocking layer to under the light-emitting surface, and diffused by the current diffusion layer to a whole area directly under the light-emitting surface. However, a sneak current path from the anode to the cathode without passing through an active layer of the light-emitting portion, which does not contribute to light emission, is likely to be formed in a conductive layer in each mesa-etched groove separating the light-emitting portions, resulting in decrease in the light output of the light-emitting diode.